Anodic bonding is a method of bonding glass to silicon by bringing glass containing alkali metal typified by borosilicate glass into contact with silicon, heating the glass and silicon to a temperature at which alkali metal ions such as a sodium ion in the glass can easily move, connecting a silicon side to a positive electrode and a glass side to a negative electrode, and applying a D.C. voltage of about hundreds to thousands of bolts. Non-cross-linked oxygen ions generated when the alkali metal ions move to the negative electrode side and silicon attract each other electrostatically to cause a chemical bond at a glass-silicon interface, thereby strong and highly-reliable bonding is obtained, which is frequently used for a mounting technology such as a pressure sensor and an acceleration sensor.
The MEMS has been used for sensors in automobiles, game appliances and the like after the research-and-development stage, and is entering the stage of practical use and spreading period. In order that the MEMS is mounted on communication equipment such as a cellular phone, it is considered to be important to realize the reduction in size and profile, and the enhanced performance, as well as the reduction in cost. Actually, about 80% of the cost of the MEMS is considered to be charge for mounting using a package and for an inspection, and thus, the package mounting is a serious problem in the reduction in size and profile and the enhancement of performance.
The MEMS that is currently used is mounted on a package as shown in FIG. 1, and the production process thereof is as follows.
1) The MEMS element produced in a wafer state is diced into chips.
2) A chip is taken out of the wafer and attached to a package substrate via a binding agent, and an electrode of the MEMS chip is connected to an electrode of the package via a metal wire.
3) The MEMS chip has a moving portion, so the MEMS chip is sealed air-tightly by covering the package with a lid.
According to the production method by package mounting, there area number of steps as described above, and in addition, the MEMS chip with the moving portion is handled without protection, so the MEMS chip is easily broken, which causes the degradation in yield. Further, a package is larger than a chip, which is a main factor that inhibits the reduction in size and profile.
As means for solving the above problems, wafer-level mounting has been developed, and the mounting method is composed of a process as shown in FIG. 2. If a silicon wafer with a MEMS element formed thereon and a mounting substrate can be directly attached to each other and air sealed, the assembly process is simplified greatly. The bonding not only eliminates the handling of a chip with a moving portion, which is a main factor of degrading yield, but also enables the reduction in size and profile of the MEMS. Further, the number of chips that can be obtained from one wafer increases, which has a large effect on the reduction in cost. In order to realize the wafer-level mounting, a through wiring mounting substrate for taking a signal out of the air-tightly sealed MEMS chip is required. This is because it is necessary that an electrode on the mounting substrate side bonded to the electrode of the MEMS device is led to the reverse surface. As a material for the through wiring substrate, glass or silicon with a through-hole filled with a conductive material is used.
When drilling used for opening a hole in glass is employed, the shape of a hole is satisfactory; however, there is a limit in the reduction in a hole size and a pitch. Further, when the number of holes in the wafer increases, it becomes difficult to grind the wafer, so there is a limit in the number of holes. Further, although a number of holes can be concurrently formed by sandblasting, the hole shape is poor and there is also a limit in the hole size and pitch. After the hole opening, a through wiring treatment is performed, which requires a complicated step. In an example put into practical use, the metallization of a hole side surface, the insertion of a metal core material for conduction, the flowing of a wax material, and the mirror polishing are performed in the stated order. Thus, sandblasting requires a high cost and is limited in miniaturization.
Silicon can be subjected to fine hole opening using a Deep RIE apparatus; however, the Deep RIE apparatus is very expensive and takes a long treatment time. A through wiring substrate is produced in complicated steps similarly to the case of glass after the hole opening. The oxidation for insulation, the formation of a seed layer for electroplating, the hole plugging by electroplating, and the mirror polishing are performed in the stated order. Further, because silicon is not anodic bondable, silicon and a silicon MEMS wafer are bonded to each other by a plasma metal activation method or bonding at normal temperature. However, the bonding apparatus is quite expensive and takes a long bonding time. Thus, although a substrate having fine holes can be produced, there is a problem in that the considerable capital investment is required, and cost is high.
As described above, materials using glass and silicon have been considered as a material for a wafer-level mounting substrate. However, with those materials, the step of forming through wiring is complicated and its cost is high, and furthermore, a multi-layered wiring cannot be formed, so there is no degree of freedom in design.
As the multi-layered wiring substrate, a low-temperature co-fired ceramics (LTCC) substrate has been known widely. The LTCC is a ceramic multi-layered substrate used frequently in high-frequency components and a module substrate for cellular phones and automobile parts. The LTCC is a substrate that can be used for producing a through wiring and multi-layered wiring with good productivity at a low cost, and furthermore, can contain passive components such as a capacitor and a coil in the substrate.
The LTCC substrate is generally produced by adding an organic binder to a mixed material, in which glass or the like is added to a ceramics material, forming the mixture into green sheets, opening through-holes in the sheet for connecting vertically, printing a paste containing a conductor into the through-holes and the surfaces of the green sheets, placing the green sheets exactly on top of the other, laminating the resultant by heating under pressure to integrate the resultant, and firing the resultant.
Holes can be punched in the LTCC substrate easily with high productivity by punching the green sheets with a punch pin or subjecting the green sheets to a laser treatment. Further, the green sheets with punched holes can be filled with the conductor easily by screen printing that is a general-purpose technology.
It is difficult to form a multi-layered wiring with glass or silicon. However, the LTCC substrate is produced by laminating a number of green sheets, so a multi-layered substrate can be obtained easily. Because wiring can be redesigned inside the layers, a degree of freedom in wiring design increases without being concerned about the electrode pad position on the MEMS chip side and the electrode pad position on the secondary mounting side. If required, as commonly-performed in the LTCC substrate, the function of the LTCC substrate may be enhanced by allowing the LTCC substrate to contain passive components such as a capacitor and a coil.
Thus, compared with glass or silicon that is a material for an MEMS through wiring electrode substrate, which is currently used, the LTCC substrate can be produced easily and reduced in cost.
However, the thermal expansion coefficient of the LTCC substrate is not matched with that of silicon, and a method of bonding to silicon is limited to a method using an inclusion such as soldering, a wax material, glass or an organic adhesive. Therefore, it has been difficult to adopt the LTCC substrate in view of the reliability in bonding to a wafer.
In order to use the LTCC substrate as a substrate for MEMS wafer-level mounting, it is necessary to develop a material to have a bonding technology that has not been able to be used with respect to the LTCC substrate available, and such a material is an anodic bondable material. The anodic bonding is a method used for bonding silicon to a mounting substrate made of a glass substrate. The method, being simple in production facilities, attains a high bonding yield and high reliability. The LTCC substrate and other materials are bonded to each other by a method using a material having an adhesive ability such as Au/Sn eutectic soldering in most cases. Regarding the anodic bonding, recently, only a material capable of being assembled by anodic-bonding using sodium ions as conductive ions has been disclosed in Germany (WO 2005/042426).
WO 2005/042426 describes a glass ceramics (LTCC) capable of being assembled with silicone by anodic-bonding, and specifically, a low-temperature fired ceramics using ceramic powder made of glass powder containing an alkali metal, alumina, cordierite and/or silica glass. As the glass powder, borosilicate glass containing about 2.6 wt % of Na content is used, and the coefficient of thermal expansion of the low-temperature fired ceramics is 3.4 ppm/° C. that is substantially equal to that of silicon. The low-temperature fired ceramics have a composition as follows: 60 to 70 wt % of borosilicate glass; 10 to 20 wt % of alumina; 8 to 25 wt % of cordierite or silica glass, with the Na content of 1.5 wt % or more.
A heating treatment is required for bonding a material to silicon. Therefore, if the material does not have a thermal expansion coefficient similar to that of silicon, there is a possibility that the electrode position of the MEMS wafer may be displaced from the electrode position of the mounting substrate due to heat stress. However, in the LTCC, a material having a thermal expansion coefficient similar to that of silicon has been developed merely for a special application. The LTCC material conventionally provided by the present inventors has a thermal expansion coefficient of 5.5 ppm/° C., which is smaller than 7 ppm/° C. of the high temperature co-fired ceramics (HTCC) but is not a sufficient value for use in a wafer-level mounting substrate for MEMS. The present inventors further conducted research and development, and newly provided an LTCC that can be subjected to the anodic-bonding and has a thermal expansion coefficient similar to that of silicon in 2007.
This material is composed of Na2O—Al2O3—B2O3—SiO2 based glass and ceramic powder, and the thermal expansion coefficient thereof is adjusted to 3.3 ppm/° C. which is close to the same as that of silicon. Further, Na ions are used as conductive ions during anodic bonding, and it was also confirmed that the material can be bonded under the conditions of a temperature set at 400° C. and applying a voltage of 600 VDC. While a prototype was made and investigated with this substrate, the following findings (a) to (b) were obtained.
(a) A low anodic bonding temperature is desired due to the little influence on the MEMS. It was found that the reduction in temperature can be realized by changing the conductive ions during anodic bonding from Na ions to Li ions having a smaller ion radius.
(b) While a prototype was made, it was found that a thin substrate with a large diameter is likely to crack and has an unstable handling property, and a material with high strength is required.